1. Field of the Invention
The present invention relates to a method of making a composite particle for an electrode, which becomes a constituent material of an electrode usable in an electrochemical device such as primary battery, secondary battery (lithium ion secondary battery in particular), electrolytic cell, and capacitor (electrochemical capacitor in particular), a method of making an electrode formed by using thus obtained composite particle for an electrode, and a method of making an electrochemical device equipped with the electrode. The present invention also relates to apparatus for making a composite particle for an electrode, an electrode, and an electrochemical device, which are equipped with respective mechanisms based on the methods mentioned above.
2. Related Background Art
In recent years, portable devices have been developing remarkably. Its major driving forces include developments in high-energy batteries such as lithium ion secondary batteries widely employed as power supplies for the devices.
A high-energy battery such as lithium ion secondary battery is mainly constituted by a cathode, an anode, and an electrolytic layer (e.g., a layer made of a liquid or solid electrolyte) disposed between the cathode and anode. Conventionally, the cathode and/or anode has been made by way of the steps of preparing its corresponding electrode forming coating liquid (e.g., in a slurry or paste form) containing an electrode active material therefor, a binder (synthetic resin or the like), a conductive auxiliary agent, and a dispersant and/or solvent; applying the coating liquid to a surface of a collector member (e.g., metal foil); and then drying, so as to form a layer containing the electrode active material (hereinafter referred to as “active material containing layer”) on the surface of the collector member.
In this method (wet method), there are cases where no conductive auxiliary agent is added to the coating liquid. There are also cases where a kneaded product containing the electrode active material, binder, and conductive auxiliary agent without using the dispersant and solvent is prepared in place of the coating liquid and then is formed into a sheet by using a hot roller and/or a hot press (dry method). Further, there are cases where a conductive polymer is added to the coating liquid, so as to form a so-called “polymer electrode”. When the electrolyte layer is solid, there are cases where a method of applying the coating liquid to the surface of the electrolyte layer is employed.
In the fields of high-energy batteries such as lithium ion secondary batteries and electrochemical devices such as electrochemical capacitors represented by electric double layer capacitors, various studies and developments have been under way in order to further improve characteristics in response to future developments of devices such as portable devices in which electrochemical devices are to be mounted.
In the case of high-energy batteries, improvements in battery characteristics such as higher capacity, improved safety, and improved energy density have been demanded. In order for lithium ion second batteries and the like to attain such higher performances, it is important to choose electrode materials. There are a wide variety of carbon materials as constituent materials of the anode, whereby various carbon materials ranging from highly crystalline graphite to carbonized polymers have been under study.
Electrochemical characteristics of batteries such as charging/discharging potential, reversible capacity, and cycle characteristic heavily depend on the degree of crystallinity (degree of graphitization), surface form, internal structure, surface chemical composition, and the like of the carbon material employed as the anode active material (negative electrode active material). In lithium ion secondary batteries using a carbon material in an anode active material, a SEI (Solid Electrolyte Interface) formed on the anode active material surface at the time of initial charging greatly influences characteristics. The SEI is generated by a reaction between the anode active material and an electrolytic solution. Once the SEI is formed, the reaction is restrained from advancing further, whereby lithium can be inserted between layers of graphite. However, the SEI is one of causes generating an irreversible capacity. The thermal stability concerning the safety of batteries depends on the stability of SEI. Because of a mechanism in which the SEI is formed by the reaction between the anode active material and the electrolytic solution, the SEI is greatly influenced by the amount of oxygen-containing functional groups such as carboxyl group and carbonyl group on the carbon particle surface and the surface structure of the carbon material such as surface crystallinity of carbon particles.
In order to overcome such a problem and yield favorable electrode and battery characteristics, to lower the irreversible capacity in particular, a technique in which a carbon material (after activation; activated carbon or the like) to become an electrode active material is subjected to thermal plasma processing, so as to purify the surface of the carbon material has been proposed (see, for example, Japanese Patent Application Laid-Open Nos. HEI 10-92432 and 2000-223121).
Though no detailed mechanisms have been elucidated yet, it has been known in general that electrochemical characteristics such as the reversible capacity, voltage resistance characteristic, cycle characteristic, and stability at the time of high-temperature storage are greatly influenced by the degree of crystallinity (degree of graphitization), surface form, internal structure, surface chemical composition, absorbed moisture amount, and the like of particles of the carbon material acting as the electrode active material. It has also been known that the amount of oxygen-containing functional groups such as carboxyl and carbonyl groups on the particle surface of the carbon material greatly affects the electrochemical characteristics mentioned above.
The above-mentioned technique intends to purify the surface of the carbon material to become the electrode active material by thermal plasma processing, and make the physical and chemical states of the surface attain states suitable for yielding sufficient electrochemical characteristics.
On the other hand, a lithium secondary battery positive electrode and a method of making the same have been proposed, in which a composite particle constituted by a manganese dioxide (cathode active material) particle and a carbon material powder (conductive auxiliary agent) immobilized on the surface of the manganese dioxide particle is used as an electrode material of the cathode, so as to prevent the charging/discharging capacity of the battery from decreasing because of the cathode, thereby further improving electrochemical characteristics (see, for example, Japanese Patent Application Laid-Open No. HEI 2-262243).
Also, a method of making a positive electrode mixture for an organic electrolytic solution battery has been proposed, in which a slurry, constituted by a positive electrode active material (cathode active material), a conductive agent (conductive auxiliary agent), a binder, and a solvent, having 20 to 50 wt % of a solid with an average particle size of 10 μm or smaller is prepared and is granulated by spray drying, so as to further improve characteristics such as discharging characteristic and productivity (see, for example, Japanese Patent Application Laid-Open No. 2000-40504).